Anatomy of an incised-valley fill from Late Quaternary depists of western Tuscany, Italy

The Late Quaternary succession of the Italian Tyrrhenian coast contains several paleovalleys, formed as a response to the last Quaternary sea-level fall. One paleovalley, cut by the Arno River in the Pisa area (Western Tuscany), exhibits 30 m of incision, is approximately 5 km wide, and can be traced landwards for approximately 20 km. At seaward locations, post-glacial sediments attain a thickness of about 60 m. A significantly thinner (17 m) succession of Holocene deposits is recorded on the interfluves. A chronologic control for the study succession is ensured by radiocarbon dating. Two distinct phases of incised-valley fill (IVF), fluvial and estuarine, forming a fining-upward succession with gravel-sand channel deposits overlain by heterolithic facies, are distinguished below the Holocene shoreface ravinement surface. These, corresponding to the lowstand systems tract and to the early transgressive systems tract (TST), respectively, are overlain by a suite of transgressive barrier and offshoretransition deposits (late TST), which are overlain by prograding prodelta, delta front and delta plain sediments (highstand systems tract). Detailed pollen data document an abrupt change in vegetation type at the fluvial/estuarine transition. Lowstand deposits in the lowermost IVF show evidence of a cold-climate vegetation, suggesting that fluvial deposition took place during the Last Glacial Maximum. A close relationship is observed between initial transgression and the development of mixed, broad-leaved woodland, indicating that deposition in a wave-dominated estuary occurred at the onset of the ensuing warm (interglacial) period. The close match between the vertical stacking pattern of facies and pollen distribution suggests that sedimentation in the Arno Basin was driven predominantly by combined eustatic sea-level changes and climatic variations. Identification of the unconformity at the base of the incised valley (sequence boundary) from core data is equivocal, due to superposition of lowstand fluvial deposits on similar, poorly-preserved alluvial sediments formed during the previous phase of falling sea-level, between 125 ky and 20 ky BP. By contrast, the transgressive surface has a distinctive facies and pollen signature and appear as the most readily identifiable surface within the valley fill for sequence-stratigraphic interpretation. Micropaleontological (benthic foraminifer and ostracod) data allow to refine the overall scheme of facies architecture, and enable precise positioning of the maximum flooding surface within an apparently monotonous, clay-dominated succession.